The present invention generally relates to a method of making a ferroelectric thin film, and also relates to ferroelectric capacitor, ferroelectric memory and method for fabricating the memory. More particularly, this invention relates to a method of making a ferroelectric thin film as a capacitive insulating film for a ferroelectric memory.
Recently, a ferroelectric thin film, exhibiting a spontaneous electric polarization, has been vigorously researched and developed to realize a nonvolatile memory that can operate at an even higher speed with a lower voltage applied. Among various other ferroelectric materials, low-melting metal oxides thereof, containing lead or bismuth, have often been studied widely. Specifically, layer-structured ferroelectrics, e.g., those containing bismuth, in particular, have attracted much attention.
A basic structure of a layer-structured ferroelectric is given by the following general formula:(S2O2)2+(Am−1BmO3m+1)2−where the S site is replaced with a trivalent cation of a constituent material (e.g., Bi or Tl) of the layered structure or a combination thereof; the A site is replaced with a univalent, divalent or trivalent cation of Na, Sr, Pb, Bi, Ba, Ca or La or a combination thereof; the B site is replaced with a tetravalent or pentavalent cation of Ti, Ta, Nb, Zr, Mo or W or a combination thereof; and m is an integer typically between 1 and 5. It is noted that mutually different m values may sometimes be used in combination to improve the property of the ferroelectric. For example, if a composition with m=3 is combined with a composition with m=1, then the distance between two adjacent layers in the layered structure can be changed arbitrarily.
Since the ferroelectric like this has a layered structure, the ferroelectric does not deteriorate so much even after its polarization has been reversed numerous times. Accordingly, a nonvolatile memory, including this layer-structured ferroelectric thin film, can operate with a relatively low voltage applied. In other words, the ferroelectric is effectively applicable as a material for a capacitive insulating film of a nonvolatile memory. Particularly, a layer-structured ferroelectric with its S site replaced with bismuth (which will be herein called a “bismuth layer-structured ferroelectric”) contains bismuth, which is a low-melting metal, and can be deposited at a relatively low temperature. For example, SrBi2Ta2O9 is usually heat-treated at about 800° C. To realize a semiconductor process at an even smaller feature size, however, the temperature of heat treatment should be further lowered. For instance, a stacked memory cell structure must be implemented to realize a very densely integrated memory operating at 1 megabit or more. In the implementation of the stacked memory cell structure, the heat treatment should be conducted at a temperature between 650 and 700° C. in view of the thermal resistance of a barrier layer (e.g., iridium oxide layer) provided to prevent reaction between electrode and plug.
A ferroelectric thin film is normally formed by a thermolysis process or a spin coating technique utilizing a sol-gel process. To further lower the heat treatment temperature, however, the sol-gel process is preferred, because this process positively takes advantage of chemical reactions. Specifically, in a sol-gel process, crystals are grown to form a thin film through polycondensation reactions with hydrolysis using a solution containing a strongly reactive metal alkoxide.
In a bismuth layer-structured ferroelectric thin film formed by the known sol-gel process, most of the crystals grown are eventually oriented along the c-axis. Generally speaking, most of the electric dipole moments of a bismuth layer-structured ferroelectric thin film exist in the a- and b-axis planes. In contrast, there are no dipole moments, or very small moments if any, along the c-axis. Thus, if a capacitor is made using a c-axis-oriented ferroelectric thin film like this, then the ferroelectric capacitor cannot exhibit spontaneous polarization strong enough to enable a nonvolatile memory to operate properly.